As is well known, a field bus is an industrial communications system used in data transmission systems that links a number of connected field devices, such as measuring probes (sensors), final control devices and/or drives (actuators), to a controller unit, wherein the units that provide the actual connection to the bus will be referred to below as bus components.
In a number of applications, the determinism, i.e., the predefined nature and unchangeability in the transmission of process data is more important than the actual transmission speed itself. For instance, field busses with connected users are known, in which a cyclical transmission of process data via a common transmission channel is preformed in order to transmit process data between individual users, thus sending and receiving process data such as, in particular, process input data, process output data and control data. For this purpose, protocol-specific data is read out from field devices connected to slave users during predetermined data cycles by a user acting as a master, and is written in the respective subsequent data cycles into field devices connected to slave users.
In addition, the data to be transferred in many system applications is in part safety-relevant data, so that errors in the data transmission must be recognized as early as possible and there must be a quick reaction when a error is recognized; e.g., a user or a (sub)system must be run into a safe condition. In a transmission of safety-relevant data via a bus, there are essentially six classes of errors to be taken into consideration. These relate to the repetition, the loss, the insertion, the incorrect sequence, the destruction, and the delay of safety-relevant data. The transmission of this data must therefore be secure.
To guarantee secure transmission of data, particularly safety-relevant process data, at least in such a manner that the above-cited error classes can actually be recognized if present, it is basically common practice to expand the transmitted process data with extra control data such as time stamps, user information and/or check information, e.g., CRCs (cyclic redundancy checks). It is a crucial disadvantage in this regard that the overhead to be transmitted conventionally increases sharply relative to the payload data to be transmitted, and the protocol efficiency thereby decreases. This weakness becomes particularly striking if only a small amount of safety data per user is to be transmitted, or if this takes place only infrequently.
One problem is therefore to demonstrate a way to transmit process data via a common transmission channel that guarantees the secure transmission of data if needed, without significantly influencing the protocol efficiency, even for applications in which equidistance of sampling and determinism in the transmission of data are important criteria.